In 1997 the U.S. Federal Communications Commission sold hundreds of spectrum licenses to AT&T, Inc. (T) and other carriers, raising $14.7M USD. The only problem was that the spectrum in question lay adjacent to Sirius XM Inc.'s (SIRI) satellite radio band, and it was feared that if AT&T made use of its new spectrum it would cause interference. Despite the fact that the FCC willingly sold the licenses to AT&T -- or other carriers which in turn sold it to AT&T -- it refused to authorize the use until Sirius XM and the carrier worked out a deal.

I. FCC Keeps the Good Times Rolling for AT&T

For over a decade that deal was never reached and the spectrum -- in the so-called WCS (Wireless Communications Service) band -- went unused.

But earlier this year Sirius XM and AT&T finally settled their differences after AT&T promised to set aside some of the spectrum to use as a "buffer" -- unused space between the two company's holdings, designed to prevent interference.

The FCC has approved two AT&T spectrum deals in the last three months.
[Image Source: Andrew Hinderaker]

The FCC gave the deal its blessing in October, and AT&T was officially in the WCS game. It then closed a set of deals with Comcast Corp. (CMCSA) and Horizon Wi-Com, a startup which rode the now-defunct WiMAX 4G standard, to acquire 608 more WCS licenses (enough spectrum to cover 82 percent of the population in 48).

Between the October and December approvals, AT&T now has a lot of spectrum on its hands to improve its service. But it will take a lot of work to do that. By AT&T's best estimate it will take "approximately three years" to fully leverage the new spectrum.

On the network side AT&T has to upgrade its base-stations; in some cases this may consist of a simple firmware upgrade, in other cases new hardware may be necessary to broadcast in the newly acquired chunks of WCS space.

Network upgrades will be necessary to use the new spectrum. [Image Source: Wind River blog]

On the device side, the broadcasting support is useless, if the chips in smartphones aren't designed to make use of the space. Again, here a mix of firmware and hardware (antenna, etc.) modifications will likely need to be applied in order to send and receive signals clearly on the new chunk of spectrum.

AT&T hopes to turn off its current fallback 2G network (EDGE) in 2017. It has begun the work to shuffle some 2G spectrum to 3G (HSPA+) and 4G (LTE) offerings, but again this will be a slow transition. AT&T also plans to activate "advanced LTE", which is expected to include voice-over LTE in 2013 (currently AT&T's 4G LTE network is only used for data traffic).

The carrier is the nation's second largest and twentieth largest carrier in the world, serving approximately 100.7M USD device-owning Americans.

It's still problematic. You're listing the band (WCS) and one of the two companies ATT has bought WCS spectrum from (Comcast but not Horizon Wi-Com). Either both companies, or neither would be a better option.